1. Field of the Invention
The present invention relates to an optical CT apparatus and image reconstructing method for projecting light to a part of an object to be measured such as an organism or the like, and calculating a spatial distribution of a characteristic amount concerning an optical characteristic of the part of object to be measured from a characteristic amount concerning an optical characteristic of the light transmitted therethrough.
2. Related Background Art
X-ray CT, ultrasonic CT, MRI, and the like have currently been in use as clinical image diagnosing apparatus. In addition, attention has recently been given to optical CT because of the fact that near infrared light exhibits a high transmissivity with respect to biological tissues, that it can measure the oxygen concentration in biological tissues, that it is safer than X-rays and the like, and so forth.
An optical CT apparatus is mainly composed of a light-projecting section for projecting light to each location of a part of an object to be measured; a light-detecting section for measuring the intensity of light projected from the light-projecting section and transmitted through the part of object to be measured; and an arithmetic section for reconstructing an absorption coefficient distribution image within the part of object to be measured from thus measured optical intensity and optical path.
Known as an example of image reconstructing methods is the following one by R. L. Barbour et al (xe2x80x9cImaging of Multiple Targets in Dense Scattering Mediaxe2x80x9d (H. L. Graber, J. Chang, R. L. Barbour, SPIE Vol. 2570, p. 219-p. 234)). Namely, this is a method in which light beams are projected from a plurality of locations on a surface of a part of an object to be measured toward the inside of the part of object to be measured and, from the optical path length calculated in each of volume elements into which the part of object to be measured is divided, the absorption material concentration of each volume element is determined. Here, it is necessary to use another phantom having an outer shape identical to that of the part of object to be measured but with no absorption therein, so as to measure a standard value of the detection intensity (transmission light intensity).
The above-mentioned image reconstructing method makes it possible to determine the absorption material concentration of each volume element, whereby the absorption coefficient distribution image within the part of object to be measured can be reconstructed by displaying thus determined concentration as a grayscale image, for example.
On the other hand, Japanese Patent Application Laid-Open No. HEI 6-129984 discloses that a medium (hereinafter referred to as optical interface member) having a refractive index and a scattering coefficient which are substantially the same as those of the part of object to be measured is interposed between the light-projecting section and the part of object to be measured, so as to prevent light from being reflected, scattered, and so forth on the surface of the part of object to be measured, thereby raising the accuracy in measurement.
However, the above-mentioned image reconstructing method and the optical CT apparatus using the above-mentioned image reconstructing method have problems as follows.
First, in the above-mentioned image reconstructing method, it is necessary to prepare another phantom having an outer shape identical to that of the part of object to be measured but with no absorption therein in order to measure a standard value of the detection intensity. As a consequence, a different phantom must be prepared when measuring a different part of object to be measured, which remarkably increases the measurement time. When the part of object to be measured has a complicated structure such as an organism, preparing such a phantom is difficult in terms of measurement accuracy, and thus is unrealistic.
Second, since the above-mentioned image reconstructing method uses the optical path length calculated in each of a plurality of volume elements into which the part of object to be measured is divided, so as to reconstruct an absorption coefficient distribution image within the part of object to be measured, it is necessary, when measuring a part of an object to be measured having a different outer shape, to redivide the part of object to be measured into a plurality of minute volume elements and recalculate the optical path length in each volume element. Therefore, it takes a considerable time to reconstruct the image.
It is an object of the present invention to overcome the above-mentioned problems and provide an optical CT apparatus and image reconstructing method which can rapidly reconstruct the absorption coefficient distribution image within the part of object to be measured.
For overcoming the above-mentioned problems, the optical CT apparatus of the present invention comprises a container accommodating a light-transparent medium therein; light-projecting means for projecting light into the container from at least one part of the container; light-detecting means for detecting the light projected by the light-projecting means in at least one part of the container; and arithmetic means for calculating a spatial distribution of a characteristic amount of a part of an object to be measured according to a comparison of a characteristic amount concerning an optical characteristic of the light transmitted through the medium actually measured by use of the light-projecting means and the light-detecting means in a state where the medium is accommodated within the container with a characteristic amount concerning an optical characteristic of the light transmitted through the medium and/or the part of object to be measured actually measured by use of the light-projecting means and the light-detecting means in a state where the medium is partly replaced by the part of object to be measured.
Also, for overcoming the above-mentioned problems, the image reconstructing method of the present invention comprises a first measurement step of projecting light from at least one part of a container accommodating a light-transparent medium therein into the container by using light-projecting means, and detecting the light projected by the light-projecting means in at least one part of the container by using light-detecting means, so as to obtain a characteristic amount concerning an optical characteristic of the light transmitted through the medium; a second measurement step of projecting light from at least one part of the container into the container by using the light-projecting means in a state where the medium accommodated within the container is partly replaced by a part of an object to be measured, and detecting the light projected by the light-projecting means in at least one part of the container by using the light-detecting means, so as to obtain a characteristic amount concerning an optical characteristic of the light transmitted through the medium and/or the part of object to be measured; and an arithmetic step of calculating, according to a comparison of the characteristic amount concerning the optical characteristic of the transmitted light obtained by the first measurement step with the characteristic amount concerning the optical characteristic obtained by the second measurement step, a spatial distribution of a characteristic amount concerning an optical characteristic of the part of object to be measured.
When a container having a predetermined form is used, and a spatial distribution of a characteristic amount concerning an optical characteristic of a part of an object to be measured is calculated according to a comparison of a characteristic amount concerning an optical characteristic of transmitted light measured in a state filled with a medium with a characteristic amount concerning an optical characteristic of transmitted light measured in a state where the medium is partly replaced by the part of object to be measured as in the configuration mentioned above, it becomes unnecessary to prepare another phantom having a shape identical to that of the part of object to be measured but with no absorption therein so as to prepare a standard amount. Also, it becomes unnecessary to redivide a part of an object to be measured having a different outer shape into minute volume elements and recalculate the optical path length in each volume element. As a result, the measurement time can greatly be shortened, so that the absorption coefficient distribution image within the part of object to be measured can rapidly be reconstructed.